Zinc selenide

Zinc selenide is the inorganic compound with the formula ZnSe. It is a lemon-yellow solid although most samples have a duller color due to the effects of oxidation. It is an intrinsic semiconductor with a band gap of about 2.70 eV at 25 C, equivalent to a wavelength of 459 nm. ZnSe occurs as the rare mineral stilleite, named after Hans Stille.

Synthesis and properties
ZnSe is available in both hexagonal (wurtzite) and cubic (zincblende) polymorphs. In both cases, the Zn2+ and Se2− sites are tetrahedral. The difference in the structures related to close packing motifs, hexagonal vs cubic.

Cubic ZnSe is produced by treatment of an aqueous solution of zinc sulfate with hydrogen selenide:
 * ZnSO4 + H2Se  ->  ZnSe  +  H2SO4

Heating the cubic form gives hexagonal ZnSe.

An alternative synthesis involves heating a mixture of zinc oxide, zinc sulfide, and selenium:
 * 2 ZnO + ZnS  +  3 Se  ->  3 ZnSe + SO2

It is a wide-bandgap semiconductor of the II-VI semiconductor group (since zinc and selenium belong to the 12th and 16th groups of the periodic table, respectively). The material can be n-type doped with, for instance, halogen elements. P-type doping is more difficult, but can be achieved by introducing gallium.

Applications

 * ZnSe is used to form II-VI light-emitting diodes and diode lasers. It emits blue light.
 * ZnSe doped with chromium (ZnSe:Cr) has been used as an infrared laser gain medium emitting at about 2.4 μm.
 * It is used as an infrared optical material with a remarkably wide transmission wavelength range (0.45 μm to 21.5 μm ). The refractive index is about 2.67 at 550 nm (green), and about 2.40 at 10.6 μm (LWIR). Similar to zinc sulfide, ZnSe is produced as microcrystalline sheets by synthesis from hydrogen selenide gas and zinc vapour. Another method of producing is a growth from melt under excessive pressure of inert gas (Ar usually). When free of absorption and inclusions it is ideally suited for CO2 laser optics at 10.6 μm wavelength. It is thus a very important IR material. In daily life, it can be found as the entrance optic in the new range of "in-ear" clinical thermometers, seen as a small yellow window. Zinc selenide can slowly react with atmospheric moisture if poorly polished, but this is not generally a serious problem. Except where optics are used in spectroscopy or at the Brewster angle, antireflection or beamsplitting optical coatings are generally employed.
 * ZnSe activated with tellurium (ZnSe(Te)) is a scintillator with emission peak at 640 nm, suitable for matching with photodiodes. It is used in x-ray and gamma ray detectors. ZnSe scintillators are significantly different from the ZnS ones.

Reactions
ZnSe is insoluble in water, but dissolves in concentrated hydrochloric acid.

It can be deposited as a thin film by chemical vapour deposition techniques including MOVPE and vacuum evaporation.